Method and apparatus for short term inspection or long term structural health monitoring

ABSTRACT

A method and apparatus is shown for implementing magnetostrictive sensor techniques for the nondestructive short term inspection or long term monitoring of a structure. A plurality of magnetostrictive sensors are arranged in parallel on the structure and includes (a) a thin ferromagnetic strip that has residual magnetization, (b) that is coupled to the structure with a couplant, and (c) a coil located adjacent the thin ferromagnetic strip. By a transmitting coil, guided waves are generated in a transmitting strip and coupled to the structure and propagate along the length of the structure. For detection, the reflected guided waves in the structure are coupled to a receiving strip and are detected by a receiving magnetostrictive coil. Reflected guided waves may represent defects in the structure.

This is a continuation-in-part patent application depending from U.S.patent application Ser. No. 09/815,219, filed Mar. 22, 2001, which is acontinuation-in-part patent application depending from U.S. patentapplication Ser. No. 09/519,530, filed Feb. 25, 2000, now U.S. Pat. No.6,294,912, which depends on provisional Patent Application Ser. No.60/124,763, filed on Mar. 17, 1999.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to methods and devices for shortterm inspection of structures, or long term monitoring of the health ofa structure. The present invention relates more specifically to amagnetostrictive sensor based system for short term inspection or longterm monitoring of the health of a structure.

2. Description of the Related Art

Magnetostrictive effect refers to the phenomena of a physical dimensionchange in ferromagnetic materials that occurs through variations inmagnetization. In magnetostrictive applications, the generation anddetection of mechanical waves is typically achieved by introducing apulse current into a transmitting coil adjacent to a ferromagneticmaterial. The change in magnetization within the material located nearthe transmitting coil causes the material to change its length locallyin a direction parallel to the applied field. This abrupt localdimension change, which is the magnetostrictive effect, generates amechanical wave (called guided wave) that travels through theferromagnetic material with a certain fixed speed (which is usually lessthan the speed of sound). When the mechanical wave is reflected backfrom the end of the ferromagnetic material, or from a defect in theferromagnetic material, and reaches a detection coil, the mechanicalwave generates a changing magnetic flux in the detection coil as aresult of the inversed magnetostrictive effect. This changing magneticflux induces an electric voltage within the detection coil that isproportional to the magnitude of the mechanical wave. The transmittingcoil and the detection coil can be identical.

Advantages of using the magnetostrictive effect in nondestructiveevaluation (NDE) applications include (a) the sensitivity of themagnetostrictive sensors, (b) durability of the magnetostrictivesensors, (c) no need to couple the sensor to the material beinginvestigated, (d) long range of the mechanical waves in the materialunder investigation, (e) ease of implementation, and (f) low cost ofimplementation.

The use of magnetostrictive sensors (MsS) in the nondestructiveevaluation (NDE) of materials has proven to be very effective incharacterizing defects, inclusions, and corrosion within various typesof ferromagnetic and non-ferromagnetic structures. A MsS launches ashort duration (or a pulse) of guided waves in the structure underinvestigation and detects guided wave signals reflected from anomaliessuch as defects in the structure. Since guided waves can propagate longdistances (typically 100 feet or more), the MsS technique can inspect aglobal area of a structure very quickly. In comparison, otherconventional NDE techniques such as ultrasonics and eddy current inspectonly the local area immediately adjacent to the probes used. Therefore,the use of magnetostrictive sensors offers a very cost effective meansfor inspecting large areas of steel structures such as strands, cables,pipes, and tubes quickly with minimum support requirements such assurface preparation, scaffolding, and insulation removal. The ability touse magnetostrictive sensors with little preparation of the object underinspection derives from the fact that direct physical contact betweenthe sensors and the material is not required.

Efforts have been made in the past to utilize magnetostrictive sensortechnologies in association with the inspection of both ferromagneticand non-ferromagnetic materials. Included in these efforts are systemsdescribed in U.S. Pat. Nos. 5,456,113; 5,457,994; and 5,501,037, whichare each commonly owned by the assignee of the present invention. Thedisclosures of U.S. Pat. Nos. 5,456,113; 5,457,994; and 5,501,037,provide background on the magnetostrictive effect and its use in NDE andare therefore incorporated herein by reference. These efforts in thepast have focused primarily on the inspection of pipe, tubing and steelstrands/cables wherein the geometry of the structure is such that thecross-sectional diameter is small in comparison to the length of thestructure. While these systems and their application to longitudinalstructures find significant applications, there are yet other structuresthat could benefit from the use of magnetostrictive based NDE.

Other efforts have been made in the past to utilize sensors that measuremagnetic flux and/or acoustic waves in structural materials. Theseefforts have included those described in the following patents:

U.S. Pat. No. 3,555,887 issued to Wood on Jan. 19, 1971 entitledApparatus for Electroacoustically Inspecting Tubular Members forAnomalies Using the Magnetostrictive Effect and for Measuring WallThickness. This patent describes a system designed to direct amechanical wave through the thickness dimension of a long tubularmember. The sensitivity of the device is limited to the directing of awavefront normal to the surface of the material under inspection andimmediately back to a sensor when reflected from an opposite wall or ananomaly.

U.S. Pat. No. 4,881,031 issued to Pfisterer, et al. on Nov. 14, 1989entitled Eddy Current Method and Apparatus for Determining StructureDefects in a Metal Object Without Removing Surface Films or Coatings.This patent describes a method for establishing localized eddy currentswithin ferromagnetic materials and recognizes the presence and effect ofa coating in order to identify and quantify corrosion beneath thecoating. As with other eddy current methods, the ability to inspect amaterial is limited to the area immediately adjacent to the sensor.

U.S. Pat. No. 5,544,207 issued to Ara, et al. on Aug. 6, 1996 entitledApparatus for Measuring the Thickness of the Overlay Clad in a PressureVessel of a Nuclear Reactor. This patent describes a system directedsolely to the measurement of magnetic field variations that result fromthe distribution of the magnetic field through overlays of varyingthickness. The system utilizes a magnetic yoke that is placed in closecontact with the surface of the overlay clad of the pressure vessel.

U.S. Pat. No. 5,687,204 issued to Ara, et al. on Nov. 11, 1997 entitledMethod of and Apparatus for Checking the Degradation of a PressureVessel of a Nuclear Reactor. This patent describes a system similar tothe earlier issued Ara, et al. patent and utilizes a magnetic yokehaving an excitation coil and a magnetic flux measuring coil that areplaced in close contact with the inner wall of the pressure vessel. Thehysteresis magnetization characteristics formed by the magnetic yoke andthe pressure vessel wall are measured. Degradation of the materialcomprising the pressure vessel is inferred from a determination of thehardness of the material which is determined from the coercive forcesobtained by analyzing the hysteresis characteristics of themagnetization.

In general, a magnetostrictive sensor consists of a conductive coil anda means for providing a DC bias magnetic field in the structure underinspection. The means for providing a bias magnetic field can includethe use of either permanent magnets or electromagnets. In a transmittingmagnetostrictive sensor, an AC electric current pulse is applied to thecoil. The resulting AC magnetic field (a changing magnetic field)produces guided waves in an adjacent ferromagnetic material through themagnetostrictive effect. For pipes, cables, tubes, and the like, thewaves are typically launched along the length of the longitudinalstructure. In the receiving magnetostrictive sensor, a responsiveelectric voltage signal is produced in the conductive coil when theguided waves (transmitted or reflected from anomalies within thematerial) pass the sensor location, through the inverse magnetostrictiveeffect.

With MsS techniques, defects are typically detected by using thepulse-echo method well known in the field of ultrasonics. Since thesensor relies on the magnetostrictive behavior found in ferrogmagneticmaterials, this technology is primarily applicable to the inspection offerromagnetic components such as carbon steel piping or steel strands.It is also applicable, however, to the inspection of nonferrouscomponents if a thin layer of ferromagnetic material, such as nickel, isplated or coupled onto the component in the area adjacent to themagnetostrictive sensors.

The magnetostrictive sensor technique has the advantage of being able toinspect a large area of material from a single sensor location. Suchsensors have, for example, been used to accurately inspect a length ofpipe or cable of significantly more than 100 feet. Further,magnetostrictive sensor techniques are comprehensive in their inspectionin that the methods can detect both internal and external defects,thereby providing a 100% volumetric inspection. The techniques are alsoquite sensitive, being capable of detecting a defect with across-section less than 1% of the total metallic cross-section ofcylindrical structures such as pipes, tubes, or rods. Finally, asindicated above, magnetostrictive sensor techniques do not requiredirect physical contact between the component surface and the sensoritself. This eliminates the need for surface preparation or the use of acouplant.

Application to Plate Type and Containment Structures

In recent years, there have been many reported occurrences of steelcontainment liners degrading at commercial nuclear power plants. Due tothe aging of such facilities and the increased requirements forinspection, incidents of degradation are likely to increase. Thestructural degradation of these liners, especially corrosion damage, isan important concern since the liners are designed to provide aleak-tight pressure boundary for the nuclear containment. Many otherindustrial uses of plate type ferromagnetic materials could benefit frommore frequent inspections to determine the state of deterioration, thelocation of faults, and the likelihood of failure. In most instances inthe past, inspections of large plate type objects (such as largeaboveground storage tanks) have required either very expensive off-lineinspections or statistical samplings of randomly selected local areasthat are for the most part less than reliable. It has heretofore beendifficult to carry out a thorough inspection of a plate type structure,or a structure comprised of a plurality of plate type sheets ofmaterial, without high cost and long down time for the object underinspection. It would be desirable to use the magnetostrictive sensortechnique for detecting and locating various anomaly characteristicswithin plate type materials. Such techniques could be used for detectingand locating wall thickness reductions in liners, such as thosedescribed above, that might be caused by corrosion over time. If such asystem were applicable, it would be possible to inspect otherwiseinaccessible regions of containment liners and the like that are eitherimbedded in concrete or adjacent to flooring or equipment that cannot bemoved.

It would therefore be desirable to implement magnetostrictive sensortechniques in conjunction with plate type structures in a manner similarto, and with the accuracy of, such systems utilized in conjunction withcylindrical structures. It would be desirable if an inspection of platetype and cylindrical structures could be carried out in an efficientmanner that did not require full access to the surface of the plate orthe inner or outer surface of cylindrical structures such as pipes andtubes. Such a magnetostrictive sensor system would be able toinvestigate large volumes of a plate type or cylindrical structure,including pipes and tubes, and would provide a cost effective globalinspection of the structure.

SUMMARY OF THE PRESENT INVENTION

It is therefore an object of the present invention to provide a sensordevice for implementing magnetostrictive based NDE in association withpipes and tubes in order to evaluate the condition of the structures andto determine the presence of anomalies indicative of fractures,deteriorations, and the like.

It is a further object of the present invention to provide amagnetostrictive sensor appropriate for use in conjunction with theinspection of pipes and tubes that is capable of transmitting andreceiving guided waves within the pipes and tubes and generating signalsrepresentative of the characteristics of such waves appropriate for theanalysis and detection of anomalies therein.

It is a further object of the present invention to providemagnetostrictive sensor devices appropriate for use in conjunction withthe inspection of pipes and tubes that inspect the entire structure foranomalies, corrosion, fractures, and the like in a cost effectivemanner.

It is a further object of the present invention to provide a method forthe inspection of pipes and tubes that includes the use of amagnetostrictive sensor specifically adapted for directing guided wavesalong the length of the pipe or tube and detecting such waves as may bereflected from anomalies along the pipe or tube.

It is yet another object of the present invention to provide a methodand apparatus for nondestructive evaluation of pipes and tubes utilizingmagnetostrictive sensors that generate and detect shear horizontal wavesalong the length of the item being inspected.

It is yet another object of the present invention to provide amagnetostrictive sensor that is suitable for low frequency operation(200 kHz or less), has good sensitivity and long inspection range, andis relatively tolerate to liftoff.

It is still another object of the present invention to provide a methodand apparatus for nondestructive evaluation of pipes usingmagnetostrictive sensors that propagate guided waves in acircumferential direction around the pipe.

Another object of the present invention is to provide a method andapparatus for nondestructive evaluation of pipes and tubes usingmagnetostrictive sensors with torsional waves that has better defectdetectability particularly in liquid filled pipes or tubes.

Still another object of the present invention is to provide a method andapparatus for the nondestructive evaluation of pipes and tubes thatrequires no permanent DC bias magnets or electromagnets and, thus iseasier to apply.

Another object of the present invention is to provide a method andapparatus for the nondestructive evaluation of pipes and tubes that hasa reduced setup time and therefore a lower inspection cost.

In fulfillment of these and other objectives, the present inventionprovides a method and apparatus for implementing magnetostrictive sensortechniques for the nondestructive evaluation of plate type structuressuch as walls, vessels, enclosures, and the like. The system includesmagnetostrictive sensors specifically designed for application inconjunction with welded plate type structures that generate guided wavesin the plates which travel through the plate in a direction parallel tothe surface of the plate. Similarly structured sensors are positioned todetect the guided waves (both incident and reflected) and generatesignals representative of the characteristics of the guided wavesdetected. The system anticipates the use of either discretemagnetostrictive transmitters and receivers or the use of a singlemagnetostrictive sensor that operates to both transmit and detect theguided waves. The sensor structure is longitudinal in nature andgenerates a guided wave having a wavefront parallel to the longitudinaldirection of the sensor. Appropriate electronics associated with theprocess of generating the guided waves and controlling the propagationdirection of the generated wave through the magnetostrictive transmitteras well as detecting, filtering, and amplifying the guided waves at themagnetostrictive receiver, are implemented as is well known in the art.Signal analysis techniques, also known in the art, are utilized toidentify anomalies within the plate type structure. The method utilizespattern recognition techniques as well as comparisons between signalsignatures gathered over time from the installation of the structureunder investigation to a later point after deterioration and degradationmay have occurred.

The magnetostrictive sensors can also be used to detect defects incylindrical structures such as to detect defects in electric resistancewelding, such as in pipes that are welded along a seam thereof. Forexample, a magnetostrictive transmitter can be placed on one side of thepipe being investigated and a magnetostrictive receiver on the otherside of the pipe. By propagating a guided wave in circumferentialdirection around the pipe, any defects in the pipe can immediately bedetected, such as in the area of the weld.

For generation and detection of the symmetrical (S) or theanti-symmetrical (A) Lamb wave mode in a plate type structure, the DCmagnet or field required for MsS operation is applied parallel to thedirection of wave propagation. For generation and detection of the shearhorizontal (SH) wave mode, the DC magnetic field required for MsSoperation is applied perpendicular to the direction of wave propagation.Due to the enclosed nature of cylindrical structures such as pipes andtubes, the shear horizontal wave can be induced to act as a torsionalwave along the length of the pipe or tube. The generation of a shearhorizontal or torsional wave along the length of the pipe or tube allowsdefect detectability that will not be hampered by the presence of liquidin the pipe or tube.

Current flow along the longitudinal axis of a pipe or tube will causemagnetization of a ferromagnetic pipe or tube in the circumferentialdirection. This magnetization can be used for the transmission anddetection of torsional waves that flow along the pipe and tube and anyreflections thereof. The reflections may be from anomalies or defects inthe pipe or tube.

Also, a thin ferromagnetic strip that is magnetized in thecircumferential direction may be wrapped around and held tightly againstthe pipe or tube. Thereafter, a torsional wave may be generated ordetected where the ferromagnetic strips are located along the pipe ortube. The circumferential magnetization around the pipe or tube is inthe ferromagnetic strip. It is very important to hold the ferromagneticstrip in tight surface contact with the pipe or tube so that the fulleffect of the torsional wave can be felt and detected in either thetransmitter or receiver coils adjacent thereto.

In another embodiment of the present invention, a thin strip of aferromagnetic material that can retain residual magnetization, such asnickel, is prepared an appropriate width and length. The width of thestrip depends upon the operating frequency of the magnetostrictivedevice. The length of the strip depends upon the structure to bemonitored. For example, for a pipe, the length is slightly shorter thanthe circumference of the pipe. For a plate-type structure, the length istypically 10 inches or less.

Residual magnetization is induced along the length of the strip byapplying an external magnetic field to the strip along its length andthen removing the external magnetic field. Afterwards, the strip iscoupled to the structure to be monitored with an appropriate material,such as epoxy. For a pipe, the strip is bonded around the circumferenceof the pipe. For a plate-type structure, the strip is bonded normal tothe direction of wave propagation to be used for inspection. For shortterm inspection, a viscous couplant, such as honey, may be used tocouple the strip on a temporary basis to the structure being inspected.

After coupling the strip to the structure to be inspected, a coil iseither wrapped around or placed adjacent to the magnetized strip. Aminimum of two strips and coils are used, one for transmitting and onefor receiving the magnetostrictive signals.

For long term monitoring, the transmitters and receivers are encased orcovered in a manner to protect them from the environment. Wires from themagnetostrictive transmitters and receivers are easily accessiblewhereby the transmitters and sensors can be electrically monitored byappropriate magnetostrictive instrumentation.

For short term inspection, the signal obtained indicates if there is adefect in the structure being inspected. For long term monitoring, abaseline signal is obtained and stored in the computer. Thereafter,additional signals are obtained periodically from the magnetostrictivetransmitters and receivers with changes in the signal indicating changesin the structure being inspected.

The guided wave normally used in the method just described for pipingapplications is a torsional wave and in plate-type structures is a shearhorizontal wave.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram showing the components of the systemof the present invention.

FIG. 2 is a perspective view of a magnetostrictive sensor of the presentinvention.

FIG. 3 is a cross-sectional view of the implementation of the sensors ofthe present invention in conjunction with a plate type structure.

FIG. 4 is a plot of a signal received through the system of the presentinvention utilizing a 60 kHz symmetric (S₀) wave mode signal in a 4 footwide, 20 foot long, 0.25 inch thick steel plate.

FIG. 5 is a plot of a signal received through the system of the presentinvention in conjunction with the structure associated with FIG. 4 for a40 kHz anti-symmetric (A₀) wave mode signal.

FIG. 6 is a plot of three signals received through the system of thepresent invention utilizing a 40 kHz symmetric (S₀) wave mode signal ina 4 foot wide, 20 foot long, 0.25 inch thick steel plate.

FIG. 7 is a plot of three signals received through the system of thepresent invention utilizing a 20 kHz anti-symmetric (A₀) wave modesignal in a 4 foot wide, 20 foot long, 0.25 inch thick steel plate.

FIGS. 8(a) and (b) are plots of a shear horizontal (SH) wave receivedthrough the system of the present invention utilizing an 80 kHz wave ina 4 foot wide, 20 foot long 0.25 inch thick steel plate, before andafter a 0.05 inch hole is cut therein.

FIG. 9 is a pictorial end view of a welded pipe being inspected using amagnetostrictive transmitting probe and a magnetostrictive receivingprobe on opposite sides of the pipe for transmission and receipt of Lambor SH waves.

FIGS. 10(a) and (b) are plots of signals received through the system ofthe present invention when used to test a pipe as shown in FIG. 9,utilizing a 150 kHz SH wave mode in a 4.5 inch outside diameter steelpipe having a 0.337 inch thick wall before and after cutting a notchtherein.

FIG. 11 is a pictorial view of a pipe being inspected using amagnetostrictive transmitting probe and a magnetostrictive receivingprobe for transmission and receipt of torsional waves with a high DCelectric current for circumferential magnetization.

FIG. 12 are plots of torsional wave signals received through the systemof the present invention depicted in FIG. 11 when used to test a pipefilled with water, utilizing a 32 kHz torsional wave mode in a 4.5 inchoutside diameter steel pipe having a 0.337 inch thick wall and 168 footlength.

FIG. 13 is an illustration of different types of magnetostrictive wavesin a plate to illustrate dimensional changes in the plate.

FIG. 14 is a cross-sectional view of a transmitter or receiver attachedto a pipe for transmission or receipt of torsional waves.

FIG. 15 is another embodiment of a cross-sectional view of a transmitteror receiver attached to a pipe for transmission or receipt of torsionalwaves.

FIG. 16 is a plot of a signal received using the embodiment as shown inFIG. 14 on a 9.3 foot long pipe having 4 inch outside diameter and a0.224 inch thick wall, with the transmitters and receivers being locatedon each end of the pipe.

FIG. 17 is yet another embodiment of a cross-sectional view of atransmitter and receiver attached to a tube for transmission or receiptof torsional waves from inside the tube.

FIG. 18 is another embodiment of a cross-sectional view of a transmitteror receiver attached to a pipe for transmission or receipt of torsionalwaves.

FIG. 19 is a pictorial diagram showing a use of the present inventionfor long term monitoring of a pipe.

FIG. 20 is a pictorial view of the present invention as used on a testpipe.

FIG. 21 is plots of signals received using the test pipe as shown inFIG. 20, using a differential algorithm.

FIG. 22 is plots of signals received using the test pipe as shown inFIG. 20, using another type of differential algorithm.

FIG. 23 is a cross-sectional view of the embodiment shown in FIG. 18 asapplied to a plate.

FIG. 24 is a top view of FIG. 23.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

As indicated above, the present invention utilizes the basicmethodological approach of earlier developed magnetostrictive sensortechniques associated with the inspection of cylindrical structures suchas pipe, tubes, and the like. The basic system of such techniques iscombined with a novel magnetostrictive sensor for application to platetype structures. Reference is made first to FIG. 1 for a generaldescription of the complete system utilized to carry on the inspectionof a plate type structure. Inspection system 10 includes amagnetostrictive sensor transmitter control 12 and an associatedtransmitter coil/core 14. Transmitter coil/core 14 is positionedadjacent to the surface of plate type structure 34. Also positioned nearthe surface of plate type structure 34 is receiver coil/core 20.Receiver coil/core 20 is positioned to detect reflected waves withinplate type structure 34 and to thereby generate a signal representativeof the wave characteristics that are reflected from a defect present inthe structure. Receiver coil/core 20 is connected to preamp/filter 18which in turn is connected to computer system 16.

Magnetostrictive sensor transmitter control 12 is comprised of functiongenerator 22, power amplifier 24, and synchronization circuitry 26.These elements together generate an appropriate signal for drivingtransmitter coil/core 14 and thereby generate guided waves within platetype structure 34.

Computer system 16 is comprised of memory 28, digital processor 30, andanalog to digital converter 32. These components together receive,digitize, and analyze the signal received from receiver coil/core 20.The signal contains wave characteristics indicative of thecharacteristics of the reflected guided waves present in plate typestructure 34.

Both transmitter coil/core 14 and receiver coil/core 20 have associatedwith them bias magnets 36 and 38, respectively. Bias magnets 36 and 38are positioned adjacent the coils/cores 14 and 20 near plate typestructure 34 in order to establish a bias magnetic field to facilitateboth the generation of guided waves within structure 34 and theappropriate detection of reflected guided waves.

Reference is now made to FIG. 2 for a detailed description of the novelmagnetostrictive sensor structure utilized in the present invention.Magnetostrictive sensor 11 as shown in FIG. 2 could be utilized aseither transmitter coil/core 14 or receiver coil/core 20 described abovein FIG. 1. Magnetostrictive sensor 11 is comprised of a plurality ofU-shaped cross-sectional cores stacked in a lengthwise direction to forma sensor with a longitudinal axis that is long in comparison to itscross-section. Core elements 15 a through 15 n in the preferredembodiment may be made from a stack of U-shaped ferrites, transformersteel sheets, mild steel, or permanent magnets. The core elements 18 athrough 15 n could have other shapes; however, U-shaped or E-shaped coreelements have been found to be more efficient. If an E-shaped core isused, a transmitter may be located on one part of the E with a receiveron the other part of the E.

Surrounding the stack of U-shaped cores 15 a through 15 n is wire coil17. The number of turns for coil 17 is dependent upon the drivingcurrent and the magnetic permeability of core 15 and may be varied as iswell known in the art.

FIG. 3 shows in cross-sectional view the application of a pair ofsensors structured as shown in FIG. 2 and implemented in conjunctionwith the methods of the present invention. In FIG. 3, a cross-section ofplate type structure 34 is shown with transmitter coil/core 14 andreceiver coil/core 20 positioned on the plate. The view in FIG. 3 ofboth transmitter coil/core 14 and receiver coil/core 20 iscross-sectional in nature in order to show the establishment of amagnetic flux within plate type structure 34. Associated with each ofthe coils/cores 14 and 20 are bias magnets 36 and 38. In FIG. 3, biasmagnets 36 and 38 are shown placed over coils/cores 14 and 20. It isunderstood that in the actual implementation of the present invention,bias magnets 36 and 38 may be one or two magnets. What is necessary isthat a magnetic field be generated in plate type structure 34 under thetransmitter coil/core 14 and the receiver coil/core 20. It is onlycritical that the DC bias magnetic fields established by bias magnets 36and 38 are established within the volume of plate type structure 34under transmitter coil/core 14 and under receiver coil/core 20 asappropriate.

Transmitter coil/core 14 is comprised of core material 40 and coilwindings 42. Together these components, as driven by themagnetostrictive sensor transmitter control (not shown), operate togenerate changes in the magnetic field established by bias magnet 36within plate type structure 34. This time-varying or AC magnetic fieldwithin plate type structure 34 generates a guided wave that propagatesin a direction parallel to the surface of plate type structure 34. Thisguided wave is depicted as wave 50 in FIG. 3 and propagates in adirection away from transmitter coil/core 14. If, as shown in FIG. 3,transmitter coil/core 14 is placed on the surface of plate typestructure 34, with the longitudinal axis of coil/core 14 directed intothe drawing page in the view shown, wave 50 would propagate in twodirections away from the longitudinal axis of coil/core 14 and throughplate type structure 34. This would serve to investigate the volume ofplate type structure 34 bounded by the length (long axis) of themagnetostrictive sensor utilized. In this manner, an inspection “sweep”of a volume of plate type structure 34 can be carried out generallyequal in width to the length of the magnetostrictive sensor.

The arrangement of the magnetostrictive sensor utilized as the detectioncoil in the present invention is essentially the same as the arrangementfor the transmitter coil. In FIG. 3, receiver coil/core 20 is comprisedof core material 44, shown in cross-section, as well as coil windings46. Bias magnet 38 is likewise positioned over receiver coil/core 20.This arrangement establishes a bias magnetic field within plate typestructure 34 that fluctuates according to the presence of reflectedguided waves within the material adjacent the sensor. In FIG. 3,reflected guided waves are depicted as 52 proximate to receivercoil/core 20 and are detected thereby. In this manner, guided wavespassing through plate type structure 34 under receiver coil/core 20 aredetected and “translated” into voltage fluctuations in coil 46 in amanner that generates an appropriate signal for analysis by the balanceof the electronics of the system of the present invention (not shown).

As indicated above, the methods and apparatus of the present inventioncan be utilized in conjunction with discrete magnetostrictivetransmitters and receivers or in conjunction with a singlemagnetostrictive sensor operable as both a transmitter and a receiver.In the latter case, the structures described in FIG. 3 would be limitedto a single magnetostrictive sensor of the configuration shown foreither transmitter coil/core 14 or receiver coil/core 20.

In another alternative approach, one with greater practical application,two transmitter sensors and two receiver sensors may be used when thesensors are controlled by appropriate phasing. In this manner, thedirection of the interrogating beam may be controlled. As an example,when the transmitter generates the wave in a first position (+)direction, the return signals may be detected by a receiver controlledto detect waves traveling in the negative (−) direction. As mentionedabove, this control is achieved by phasing the two sensorsappropriately, a process well known in the field of NDE techniques. Inthis manner, an inspection of the plate may be carried out first to oneside of the transmitting sensor and then by simply switching the sensorinstrumentation an inspection may be carried out to the opposite side ofthe transmitting sensor. Various other inspection techniques known andused with magnetostrictive sensors may likewise apply with the methodsand structures of the present invention.

Reference is now made to FIGS. 4 and 5 for a detailed description ofsample data acquired from a 0.25 inch thick, 20 foot long, and 4 footwide steel plate investigated by the devices and methods of the presentinvention.

The signal represented in FIG. 4 shows the first symmetric wave mode(S₀) in the plate while the signal depicted in FIG. 5 shows the firstanti-symmetric wave mode (A₀). FIG. 4 is a time varying amplitude plotof a 60 kHz magnetostrictive sensor signal taken from the abovedescribed steel plate geometry. The wave is directed through appropriateorientation of the sensor and propagates in the long direction withinthe steel plate. The signal components identified in FIG. 4 include theinitial pulse 60, end reflected signal 62, and trailing signals 64.Likewise in FIG. 5, initial pulse 70 is indicated, as are end reflectedsignals 72.

Anomalies within the path of the guided wave generated within thematerial would, as is known in the art, generate signal componentshaving amplitudes sufficient for identification within either of the twosignals shown in FIGS. 4 and 5. In this manner, characteristics ofanomalies detected within the plate type structure can be identified andlocated in the direction of wave propagation away from themagnetostrictive sensor. As is known in the art, the relative locationof an anomaly may be identified by the position of the signalcharacteristic indicative of the anomaly in time relationship with theinitial pulse (indicative of the position of the sensor) and the endreflected signals 62 and 72.

Examples of such signals are shown in FIGS. 6 and 7. FIG. 6 showspulse-echo magnetostrictive sensor data for a 40 kHz S₀ wave mode signalobtained in a 4 foot wide, 20 foot long, 0.25 inch thick steel plate.Three signals are shown for data collected with a 4 inch long, 8 inchlong, and 12 inch long notch cut in the plate at a point approximatelytwo-thirds of the length of the plate away from the sensor.

FIG. 7 shows pulse-echo magnetostrictive sensor data for a 20 kHz A₀wave mode signal obtained in a 4 foot wide, 20 foot long, 0.25 inchthick steel plate. Three signals are also shown for data collected witha 4 inch long, 8 inch long, and 12 inch long notch cut in the plate at apoint approximately two-thirds of the length of the plate away from thesensor.

In each case, the notch is not only detectable but may be characterizedas to size and position. Various signal analysis techniques may beapplied to these signals to discern and characterize other types ofanomalies found in such plate-type structures. Discrete fractures andthe like are typically identified by isolated reflected waves, whilebroad deteriorations or corrosions in the plate might be identified bygrouped waves received over a period of time. In addition, it isanticipated that signature signals of a particular plate type structuremight be acquired prior to implementation of the structure into service.In this manner subsequent signatures may be acquired periodically andcompared with the initial base line reference signature to determine thepresence of developing anomalies within the plate.

To prove the invention works, symmetric (S₀) and anti-symmetric (A₀)longitudinal wave mode signals were generated and detected using a 12inch long magnetostrictive probe such as shown in FIG. 2. To generateand detect these wave modes, the bias magnets 36 and 38 are applied inthe direction parallel to the direction of wave propagation(perpendicular to the lengthwise length of the magnetostrictive probe).The same probe as shown in FIG. 2 can be used to generate and detectshear horizontal waves in a plate by applying DC bias magnetic fields ina direction perpendicular to the wave of propagation (or parallel to thelengthwise direction of the magnetostrictive probe).

Using a 4 inch long magnetostrictive probe, a signal was induced in a0.25 inch thick, 4 foot wide, 20 feet long, steel plate. FIG. 8(a) showsthe signal as generated and reflected over time. The initial pulse 100is generated by the magnetostrictive transmitter controller 12 until itreaches the far end of the sheet and a signal from the far end 102 isreceived by the receiver coil/core 20. A signal from the near end 104 isreceived due to the imperfect directionality control of the system.

After drilling a 0.25 inch hole about two-thirds of the way down thesheet, another initial pulse 100 is sent down the sheet. Again, a signalis received from the near end 104 due to imperfect directionalitycontrol. Also, a signal 102 from the far end is received. However, now asignal 106 is received that indicates the 0.25 inch hole in the sheet.Therefore, FIGS. 8(a) and (b) in combination clearly illustrate thatshear horizontal waves can be used in the magnetostrictive inspectiontechniques and probes of the current invention. Also, themagnetostrictive testing of the large plate structures is a suitable forlow frequency operation (200 kHz or less), has good sensitivity and longrange inspection, and is relatively tolerate to liftoff. This is not thecase if the inspection technique had used other common nondestructiveevaluation techniques, such as electromagnetic acoustic transducers.

Pipes can be considered as plates that are simply bent in a circle.Pipes are literally made from sheet metal that is bent into a circle andwelded on one side thereof utilizing electric resistance welding.Magnetostrictive inspection techniques may be used to inspect such pipesas shown and explained in connection with FIG. 9, including the electricresistance welding. A pipe 200 is shown with a weld line 202. Atransmitter coil/core 14 is located on one side of the pipe 200 and areceiver coil/core 20 is located 180° on the opposite side of the largediameter pipe 200. While not shown, magnetic bias is provided adjacentto the transmitter coil/core 14 and the receiver coil/core 20. Using theinspection system 10 as shown in FIG. 1, an initial pulse 206 is startedaround the pipe as shown in FIG. 10(a). Each time the pulse passes thereceiver coil/core 20, a signal 208 is received. The signal 208 dies outover a period of time and after repeated revolutions around the pipe200.

If the transmitter coil/core 14 is 180° around the pipe 200 from thereceiver coil/core 20, the two opposite going waves add constructivelyproducing a single large amplitude signal. Once generated, the initialpulse 206 keeps revolving around the circumference of the pipe 200 untilall of its energy is dissipated. Therefore, the generated wave producessignals at regular intervals which are equal to the transient time ofthe shear horizontal wave to travel around the full circumference of thepipe 200. If there are any defects at the weld line 202, they willclearly be indicated as defect signals. If the weld line isapproximately 90° from transmitter coil/core 14, then the defect wouldbe approximately midway between the signals 208 as received by thereceiver coil/core 20.

To prove the measuring of the defects, the applicant, after measuringthe signal as shown in FIG. 10(a), cut a notch in the pipe 200. The testwas then repeated with an initial pulse 206 inducing a shear horizontalwave around the circumference of the pipe 200. Again, signals 208indicate each time the shear horizontal wave reaches the receivercoil/core 20. However, in addition, there are notch signals 210 that arecreated by a reflected signal from the notch that has been induced inthe pipe 200. The notch signal 210 increases in amplitude with timebecause each time the initial wave revolves around the pipe 200, itpasses the notch defect thereby producing a notch defect signal 210which is then added to the previous notch defect signal 210. Theincreasing of the notch signal 210 occurs for a period of time and thenit will decrease until its energy is dissipated, the same as signal 208.

It is possible to get a comparative indication as to the size of thedefect by the ratio between the first initial wave signal amplitude 208and the first defect signal amplitude 210. In the example illustrated inFIG. 10(b), the notch is approximately 8% of the cross-sectional area.This compares well to the ratio of signal 208 to 210 being approximately10%. This is intended to be a rough generalization as to the size of thenotch. Obviously, other factors would be considered, such as whether thenotch is perpendicular or parallel to the direction of travel of theshear horizontal wave.

By use of the method as just described, the present invention can beused to inspect pipes for longitudinal defects and corrosion defects. Inthe present method, the magnetostrictive probes are moved along thelength of pipe to determine any defects in the pipe. In manufacturingfacilities, the magnetostrictive transmitters or receivers may bestationary with the pipes moving therebetween and simultaneously beinginspected for any defects.

While one of the advantages of the present invention is the ability tocarry out broad inspections of large volumes of a plate type structurefrom a single positioning of the sensor, it is anticipated that thecomplete investigation of a containment vessel or the like would requiremultiple placements of the sensor in a variety of positions andorientations. For example, a containment vessel might require theplacement of the sensor in a sequential plurality of positions along apredetermined scan line (which could be either horizontal or vertical tothe floor) that best achieves the inspection of the entire structure. Inthis manner, a progressive inspection of an entire containment vessel iscarried out without the requirement that all surfaces of the vessel beaccessed.

FIG. 11 is a pictorial view of a pipe 300 being inspected using amagnetostrictive transmitter 314 and a magnetostrictive receiver 320 onthe pipe 300 for transmission and receipt of torsional waves. A currentsource 322 is applied to the pipe 300 at contact points 324 and 326 thatconnect around the entire pipe 300. The current source 322 can be eithera DC source or a low frequency AC (approximately 10 Hz).

At a given frequency, more than one longitudinal (L) wave mode can existin a pipe or tube. The defect detectability of the MsS technology hasbeen found to be hampered by the presence of extraneous wave modes thatwere produced by the MsS itself and/or by mode conversion of thetransmitted wave at geometric features in pipelines, such as welds,elbows and tees. In addition, when the pipe 300 under inspection isfilled with a liquid, the liquid interacts with the L-wave mode andcauses many extraneous signals to be produced, which can significantlydegrade defect detectability.

In order to overcome these deficiencies in detecting defects in pipes ortubes containing a liquid, a torsional wave is used for the inspection.The torsional wave is a shear wave that propagates along the length ofthe pipe 300 or tube. Because the torsional wave is a shear wave in apipe or tube, its interaction with a liquid is negligible (unless theliquid is viscous). Therefore, the defect detectability of torsionalwaves will not be hampered by the presence of liquid in the pipe 300. Inaddition, the torsional wave exists as a single mode up to aconsiderable frequency and, consequently, has minimal problems in defectdetectability due to the presence of extraneous wave modes. Thetorsional wave therefore is expected to have significantly better defectdetectability than the longitudinal wave modes.

To explain why a torsional wave would not be hampered by the presence ofliquid in pipe 300, an explanation of the dimensional changes in thematerial due to magnetization and the waves generated therefrom isprovided in conjunction with FIG. 13. Referring to FIG. 13, the largerarrows 350 shown in FIGS. 13a, b and c represent the direction ofpropagation of the wave front. Referring to FIG. 13a, the dotted lines352 give an exaggerated representation of the dimensional changes in theferromagnetic plate 354 when a shear wave is projecting in direction350. Arrows 356 represent the oscillations occurring by the dimensionalchanges illustrated by waves 352. For the purposes of illustration, thedimensional changes due to magnetization caused by waves 352 andillustrated by arrows 356 have been exaggerated.

Referring to FIGS. 13b and 13 c, Lamb waves are projecting along theferrogmagnetic plate 354. In FIG. 13b, the dimensional changes due to asymmetrical Lamb wave propagating in direction 350 is illustrated in anexaggerated form. The smaller arrows shown in FIG. 13b represent thedimensional changes of the plate 354. FIG. 13c shows an asymmetricalLamb wave that would propagate along plate 354, again with the smallarrows representing dimensional changes of the plate 354. As can be seenin FIG. 13, the dimensional changes in the Lamb waves shown in FIGS. 13band 13 c will react against any liquid contained in a pipe or container.However, the use of a shear wave or a torsional wave as shown in FIG.13a, because the dimensional change is in the same plane of the plate354, there would be no reaction or interference by the liquid containedin any pipe or container. Therefore, the shear or torsional wave is theideal waveform to use if the plate or pipe is being checked that maycontain a fluid.

As illustrated in FIG. 11, coil windings 342 and 346, transmitter 314and receiver 320 that are used in the existing MsS L-wave inspection areinstalled around pipe 300. A high ampere electric current is applied topipe 300 by current source 322 applied at contact points 324 and 326along the length of pipe 300. The electric current flowing along pipe300 sets up a DC bias magnetization in the circumferential direction ofthe pipe 300 necessary for MsS generation and detection of torsionalwaves in the wall of pipe 300. The generated torsional waves propagatealong the length of pipe 300, and signals reflected from defects in pipe300 are detected in the same manner used for L-wave pipe inspection. Theresults of experimentation on this aspect of the invention are containedin FIG. 12.

FIGS. 12a-c are plots of signals received through the system of thepresent invention utilizing torsional waves when used to test pipe 300filled with water shown in FIG. 11. The data were obtained using a 32kHz torsional wave mode in a 4.5 inch outside diameter steel pipe havinga 0.337 inch thick wall and 168 foot length. The sample containedseveral simulated defects. The DC current applied was approximately 150amps, and the frequency of the MsS was 32 kHz. Signals from smallsimulated defects (whose cross sections were about one percent of thetotal pipe 300 wall cross section) were not recognizable in these data.It is however expected that the application of a higher DC current wouldpermit detection of the small defects. The data showed no effects ofwater.

Referring to the waveform shown in FIGS. 12b and 12 c, numerals 1through 12 represent the defects that occur in the pipe. The MsStransmitter 314 and receiver 320 along with coil windings 342 and 346are located at 54 feet down the pipe 300 from one end represented by endF1. The other end of the pipe is represented by end F2. There are threewelds in the pipe represented by W1, W2 and W3, respectively, at 42feet, 84 feet, and 126 feet. When the torsional wave is propagated downthe pipe towards end F2, there will be some small amount of reflectionof the signal from end F1 because of imperfect direction control as canbe seen in FIG. 12b. Likewise, when the waveform is propagated towardsend F1, there is some reflection of the signal from end F2 as shown inFIG. 12c. Therefore, in FIG. 12b, the torsional wave signal is firstdirected towards end F2. In FIG. 12c, the signal is directed towards endF1. Also, as can be seen in the signals, some of the simulated defectsare so small they can hardly be distinguished. Other simulated defectsthat are larger in cross-sectional area can be seen in the reflectedsignals shown in FIGS. 12b and 12 c.

Referring now to FIG. 14, an alternative way of creating thecircumferential magnetic field in a pipe 400 is illustrated. Wrappedaround the pipe 400 is a ferromagnetic strip 402 that contains residualmagnetization. The ferromagnetic strip 402 would typically be about aninch wide and wrapped almost around pipe 400, with the exception of asmall gap 404 at one end thereof. The ferromagnetic strip 402 may bemade from any material that has good magnetization characteristics, suchas nickel, grain-oriented silicon steel, or a magnetostrictive material,such as TERFENDOL-D®. The objective is to have a flexible strip ofmaterial that has good magnetization characteristics (ability to retainresidual magnetization and high magnetostrictive coefficient) forwrapping around pipe 400. The residual magnetization in theferromagnetic strip 402 is induced prior to wrapping around the pipe 400by applying an external magnetic field to the ferromagnetic strip 402and then removing the external field (not shown). After wrapping theferromagnetic strip 402 around pipe 400, a magnetostrictive coil 406 isplaced around the magnetized ferromagnetic strip 402. The coil 406 maybe of the common ribbon type with a coil adapter 408 connecting the twoends of the ribbon type coil 406.

To press the magnetized ferromagnetic strip 402 against pipe 400, sometype of external pressure is necessary. The embodiment shown in FIG. 14is a flexible strap 410 wrapping around both ferromagnetic strip 402 andcoil 406. The flexible strap 410 is pulled tight by means of buckle 412,which in turn presses the ferromagnetic strip 402 against the pipe 400.The guided waves are then generated in the ferromagnetic strip 402 andcoupled into the pipe 400. For detection, the guided waves in the pipe400 are coupled to the ferromagnetic strip 402, which guided waves aresubsequently detected by the MsS coil 406 placed over the ferromagneticstrip 402.

For torsional wave generation and detection, the residual magnetizationis induced along the lengthwise direction of the ferromagnetic strip402. For longitudinal wave generation and detection, the residualmagnetization is induced along the width of the ferrogmagnetic strip402. The pressing on the ferromagnetic strip 402 provides a mechanicalcoupling of the guided waves between the pipe 400 and the ferromagneticstrip 402. The illustration as shown in FIG. 14 can be either atransmitter or a receiver of guided waves (either longitudinal ortorsional wave modes) that are propagated along the pipe 400.

Referring now to FIG. 15, another alternative is shown as to how tocreate a guided wave in pipe 500. Just as in FIG. 14, in FIG. 15, amagnetized ferromagnetic strip 502 is wrapped around the pipe 500.Again, a gap 504 will exist between two ends of the ferromagnetic strip502. Also, the same as is the case in FIG. 14, a coil 506 is wrappedaround the ferromagnetic strip 502, which coil 506 is of the ribbon typeand connected by a coil adaptor 508. However, the means of applyingpressure against the ferromagnetic strip 502 to press it against thepipe 500 is different in FIG. 15 from FIG. 14. In FIG. 15, a metal caseor container 510 encircles the ferromagnetic strip 502 and coil 506. Themetal case or container 510 is held together by clamp 512. Inside of themetal case or container 510 is located a pneumatic or hydraulic tube 514that may be inflated. By inflating the tube 514, it presses the coil 506and ferromagnetic strip 502 against the pipe 500. Again, the embodimentas just explained in conjunction with FIG. 15 may be used as either atransmitter or receiver of guided waves being propagated along pipe 500.

The width of the magnetized ferromagnetic strips 402 or 502 is adjusteddepending on the frequency and the mode of the guided waves. For highfrequencies, the magnetized ferromagnetic strips 402 or 502 should benarrower; for lower frequencies, the magnetized ferromagnetic strips 402or 502 should be wider.

The feasibility of the approach explained in FIGS. 14 or 15 has beenproven in the laboratory as illustrated in conjunction with FIG. 16.Using a 4-inch outside diameter pipe with a 0.224 inch wall thicknesspipe which was 9.3 feet long, a crude test was performed. The magnetizedferromagnetic strip 402 or 502 was made of 0.01 inch thick nickel foil.The magnetized ferromagnetic strips 402 or 502 were placedcircumferentially around each end of the pipe sample. The magnetizedferromagnetic strips 402 or 502 were mechanically coupled to the outsidesurface of the pipe and in this case strapped using the method as shownin FIG. 14. FIG. 16 shows the data acquired at 64 kHz by transmittingthe torsional wave from one end of the pipe and detecting the signals atthe other end of the pipe. The data clearly indicates FIG. 14 as beingan acceptable method for generating and detecting guided waves in pipes.

Referring to FIG. 17, a probe for generating and detecting guided wavesin a tube 600 from inside the tube 600, which uses the same principle asthe present invention, is illustrated. A pneumatic tire 602 hasferromagnetic strips 604 and 606 bonded therearound. In FIG. 17,ferromagnetic strips 604 and 606 represent a transmitter and a receiver,respectively, of the torsional waves. The pneumatic tire 602 has apressure valve 608 for inflating/deflating.

Inside of the pneumatic tire 602 are two bobbin type cores 610 and 612about which a transmitting coil 614 and receiving coil 616 are wound,respectively. To hold everything together in their respective locations,the cores 610 and 612 are mounted on rod 618.

By inflating the pneumatic tire 602 through pressure valve 608, themagnetized ferromagnetic strips 604 and 606 are pressed against theinside of tube 600. Thereafter, the guided wave generated bytransmitting coil 614 in the ferromagnetic strip 604 is coupled to thetube 600 and propagates along the tube 600. Reflected signals fromdefects in tube 600 are received back through the ferromagnetic strip606 and detected by receiving coil 616. The type of signal that will begenerated will be a guided wave that propagates along tube 600. It isenvisioned that the configuration as shown in FIG. 17 will be insertedin the end of a tube 600 to propagate a signal down the entire length ofthe tube to detect flaws or defects that may exist in the tube 600. Thecores 610 and 612 are ferrite or ferromagnetic steel to aid in thetransmission and receiving of magnetostrictive signals to and from thetube 600.

Another embodiment of the present invention that has been found usefulfor either short term inspection or long term monitoring of pipelines isillustrated in the embodiment shown in FIG. 18. A pipe 700 has a thinferromagnetic strip 702 attached to its outer surface by a suitablecouplant 704. Wrapped around the outside of the thin ferromagnetic strip702 is a coil 706 that has external connections 708 and 710.

As previously described in connection with FIG. 14, the thinferromagnetic strip 702 is about one-half inch to one inch wide and hasa gap 712 between the respective ends thereof. The thin ferromagneticstrip 702 may be made from any material that has good magnetizationcharacteristics, such as nickel, grain-oriented silicon steel or amagnetostrictive material, such as TERFENDOL-D®. The thin ferromagneticstrip 702 should have the flexibility that it can be wrapped around thepipe 700. Also, it is important that the thin ferromagnetic strip 702retain residual magnetization and have a high magnetostrictivecoefficient.

The couplant 704 may vary depending upon whether the use is for a shortterm inspection or a long term monitoring. If the use is for short terminspection, the couplant 704 would be of a thick, highly viscousmaterial, such as honey, that would stick the thin ferromagnetic strip702 to the pipe 700. Also, the coil 706 would have a coil adapter(similar to those described in connection with FIGS. 14 and 15) so thatthe coil 706 can be quickly removed. However, for the purposes of thisillustration, assume that long term monitoring is desired. For long termmonitoring, the couplant 704 would be made from a couplant that becomesa rigid material, such as epoxy, to physically bond the thinferromagnetic strip 702 to the pipe 700. For long term monitoring, it isimportant that the couplant 704 maintain a good bond with the pipe 700over an extended period of time.

The thin ferromagnetic strip 702, prior to placing on the pipe 700, hasresidual magnetization induced therein. Because a torsional wave isideal for long term monitoring of a pipe, especially a pipe that may befilled with fluid, the residual magnetization in the thin ferromagneticstrip 702 is induced in the lengthwise direction of the thinferromagnetic strip 702. Thereafter, the thin ferromagnetic strip 702 isready for bonding to the pipe 700 with the couplant 704. After bonding,the coil 706 is wrapped around the thin ferromagnetic strip 702, withthe external connections 708 and 710 being available for monitoring.

An ideal situation for the use of the magnetostrictive sensor monitoringtechnology is involving gas pipelines. It has been found that gaspipelines have a tendency to accumulate fluids inside the gas pipelinealong any low point in the gas pipeline, which fluid accumulation willtend to cause corrosion. Referring to FIG. 19, a gas pipeline 714 isburied under ground 716 so that a low point 718 exists in the gaspipeline 714. At the bottom of the low point 718 is a corrosion defect720. If there is some way to monitor the low point 718 in the gaspipeline 714, the corrosion defect 720 can be determined beforecatastrophic results, such as explosion of the pipeline.

Some distance from the low point 718 (typically up to 50 feet),magnetostrictive probes 722 (similar to those described in FIG. 18) aremounted around the gas pipeline 714. At least two magnetostrictiveprobes have to be used, but to determine directionality, a minimum offour magnetostrictive probes are necessary to make use of phased arrayinterference principals so that direction of the signals can bedetermined. In the present illustration as shown in FIG. 19, fourmagnetostrictive probes 722 are illustrated.

Because the magnetostrictive probes 722 are buried under ground 716, andmay be left buried for long periods of time with just periodicmonitoring, some type of shielding cover 724 is necessary to protect themagnetostrictive probes 722. Electrical wires 726 connect to a junctionbox 728 located at the surface 730 of the ground 716.

In actual use, periodically magnetostrictive sensor monitoringelectronics 732, similar to that described in conjunction with FIG. 1,is connected to the junction box 728 at the surface 730. Themagnetostrictive sensor monitoring electronics 732 generates a signalthat is fed through the electric wires 726 to the magnetostrictiveprobes 722 that causes a guided wave 734 to propagate along the gaspipeline 714. If there is a corrosion defect 720 in the gas pipeline714, a defect signal 736 will be reflected back to the magnetostrictiveprobes 722 for detection by the magnetostrictive sensor monitoringelectronics 732 via the electric wires 726 and connection box 728.

This system as just described in conjunction with FIG. 19 is envisionedfor use along low points of gas pipelines that need to be monitored onan infrequent basis, such as every six months. By use of a permanentreference signal and comparing future signals against the referencesignal, very small changes due to corrosion can be detected. Using thistechnique, corrosion defects as small as 0.2 percent of thecross-sectional area of the gas pipeline 714 can be detected.

This invention has been proven in the laboratory as will be explained inconjunction with FIG. 20 and the waveforms shown in FIGS. 21 and 22. Apipe 738 is shown that is 29.4 feet long and 4.5 inches in outsidediameter having a 0.337 inch thick wall, each end being represented byE1 and E2. At 4 feet from E1 are located the magnetostrictive probes722. The magnetostrictive probes 722 generate a guided wave 734 thatpropagates along the pipe 738. A corrosion defect 740 causes a reflecteddefect signal 742. The reflected defect signal 742 is 19 feet from endE1.

Referring to FIG. 21, waveform 1 shows the result of subtracting theinterference to form a waveform collected prior to any corrosion beingapplied. The reference signal subtracted from a second waveform obtainedat a time different from when the reference signal is obtained is calledthe difference signal. An initial pulse 744 is applied to the pipe 738and the directionality of the generated wave is controlled usingelectronics designed on phased array principals. The reflected endsignals (E1 and E2) are canceled out because they are in both thereference and the waveform collected before any corrosion is applied.Then after applying a defect at corrosion defect point 740 that isapproximately 0.26 percent of the cross-sectional area of the pipe wall,the difference signal is obtained for a 0.26 percent defect, the datashown in waveform 2 is obtained. This shows that the 0.26 percent defectsignal 736 is just becoming detectable. Once the corrosion defect 740 isincreased to 0.48 percent of the cross-sectional area, shown in waveform3, the defect signal 736 is clearly detectable. By increasing the sizeof the corrosion defect to 0.74 percent of the cross-sectional area, thedifference signal becomes even larger as shown in waveform 4. By thetime the corrosion defect 740 reaches 0.98 percent of thecross-sectional area as shown in waveform 5, the defect signal 736 isclearly visible.

Also as the temperature of pipe 738 varies, the signal will travel atdifferent speeds in the pipe. Therefore, when obtaining the differencesignal, sometimes there is not a perfect match in the two signals due tothe difference of speed of the signal moving along the pipe 738 causedby temperature changes. Referring to FIG. 21, the end signals E2 beginto increase in waveforms 2-5 due to the temperature change. However, forburied pipelines as illustrated in FIG. 19, the temperature undergroundis relatively constant and is close to the average mean temperature forthe area.

To give an even clearer indication as to a defect, certain processingcan be applied to the waveforms as shown in FIG. 21. For example, thesignal can be squared and then averaged over a short window of time togive waveforms 1-5 as shown in FIG. 22. In this manner, the differencesignal for the defect signal 736 is even clearer. The difference signal736 becomes detectable at slightly over 0.2 percent loss of thecross-sectional area of the pipe being monitored.

By use of the techniques as just described in conjunction with FIGS.18-22, detection of defects can occur in pipes other than ferromagneticpipes. For example, the pipe could be plastic with the torsional wavebeing transmitted to the plastic pipe through the coupling. In otherwords, the torsional wave set up in the thin ferromagnetic strip 702 istransferred to any type of pipe 700 as long as the pipe is rigid with ahigh modulus of elasticity.

The same principle can be used for plate-type structures as is shown inconjunction with FIG. 23. A plate 746 has a thin ferromagnetic strip 748coupled thereto by a couplant 750. Again, the thin ferromagnetic strip748 may be of nickel or other materials described in conjunction withFIG. 18. On top of the thin ferromagnetic strip 748 is located a platemagnetostrictive probe 752. The plate magnetostrictive probe 752 couldbe either the type illustrated in FIG. 2 or a coil laid on a printedcircuit board as illustrated in FIG. 24.

The couplant 750 is made from any thick material that will couple thethin ferromagnetic strip 748 to the plate 746. If a permanent monitoringfeature is desired, the couplant 750 would be made from a couplingmaterial that becomes rigid, such as epoxy. However, if it is desirableto periodically inspect the plate 746, and thereafter remove themagnetostrictive probe, the couplant 750 may be made from a thickviscous material, such as honey. However, other types of thick viscousmaterial that will allow the magnetostrictive probe to be removed can beused.

The guided wave to be used in conjunction with FIGS. 18-22 for pipingapplications is a torsional wave. The guided wave to be used forplate-type structures would be a shear horizontal wave. The method andapparatus as described in conjunction with FIGS. 18-23 may be used notonly to detect corrosion, but can also be used to detect transientstress signals due to vibration, cracking or mechanical impacts (forexample, crash event of a passenger car for air bag operation). By useof the system as just described, it is inexpensive to implement by theend user.

Although a description of a preferred embodiment of the apparatus andmethod of the present invention has been described, it is anticipatedthat variations in the manner in which the basic sensor structure of thepresent invention may be utilized are possible. No specific dimensionsfor the sensor structure described have been identified as such would bedependent upon the specific plate type structures to be investigated. Itis anticipated that sensors of a variety of lengths could be utilizeddepending upon the requirements of the environment of investigation. Itis anticipated that other applications of the basic sensor structuredescribed herein will be discerned by those skilled in the art ofnondestructive evaluation of materials.

What is claimed is:
 1. A method of nondestructive, short term inspectionor long term monitoring of a structure to determine if said structure(a) has a defect such as a crack, corrosion or erosion or (b) has atransient stress signal due to vibrations, cracking or mechanicalimpacts, said method comprising the following steps: preparing aplurality of thin strips of ferromagnetic material of appropriate widthand length; inducing residual magnetization along said length of saidthin strips by applying an external magnetic field and thereafterremoving said external magnetic field; coupling said thin strips inparallel to said structure; installing a magnetostrictive probe on eachof said thin strips; generating a pulse signal in a transmitter controlcircuit and delivering said pulse signal to a first of saidmagnetostrictive probes to create guided waves in a first of said thinstrips, which guided waves are coupled to said structure for propagationtherein; magnetostrictively detecting any reflected waves by a second ofsaid magnetostrictive probes in combination with a second of said thinstrips, said reflected waves being coupled from said structure to saidsecond of said thin strips; and determining if said reflected waves aredue to said defect or said transient stress signal.
 2. The method ofnondestructive, short term inspection or long term monitoring of saidstructure as recited in claim 1 wherein said coupling step includesbonding said thin strips to said structure.
 3. The method ofnondestructive, short term inspection or long term monitoring of saidstructure as recited in claim 2 wherein said guided waves are shearwaves.
 4. The method of nondestructive, short term inspection or longterm monitoring of said structure as recited in claim 2 wherein saidstructure is a pipe and said guided waves are torsional waves.
 5. Themethod of nondestructive, short term inspection or long term monitoringof said structure as recited in claim 1 wherein said coupling stepincludes using a thick, viscous material as a couplant.
 6. The method ofnondestructive, short term inspection or long term monitoring of saidstructure as recited in claim 4 wherein said determining step includesstoring a reference reflected wave and, after an appropriate period oftime, repeating said generating step and said magnetostrictivelydetecting step, comparing a second reflected wave with said referencereflected wave to determine if defects have occurred during saidappropriate period of time.
 7. The method of nondestructive, short terminspection or long term monitoring of said structure as recited in claim6 wherein said determining step includes subtracting said referencereflected wave from said second reflected wave.
 8. The method ofnondestructive, short term inspection or long term monitoring of saidstructure as recited in claim 1 wherein said determining step includesstoring a reference reflected wave, thereafter repeating said generatingstep and said magnetostictively detecting step and comparing subsequentreflected waves with said reference reflected wave to determine if saidtransient stress signal has occurred.
 9. The method of nondestructive,short term inspection or long term monitoring of said structure asrecited in claim 8 wherein said reference reflected wave is continuallyupdated.
 10. The method of nondestructive, short term inspection or longterm monitoring of said structure as recited in claim 1 wherein saidthin strip may be selected from the group of ferromagnetic materialshaving appropriate magnetostrictive coefficients consisting of nickel,grain-oriented silicon steel or TERFENDOL-D®.
 11. The method ofnondestructive, short term inspection or long term monitoring of saidstructure as recited in claim 5 wherein said couplant is honey.
 12. Anapparatus for nondestructive, short term inspection or long termmonitoring of a structure to determine if said structure (a) has adefect, such as a crack, corrosion or erosion, or (b) has a transientstress signal due to vibrations, cracking or mechanical impact, saidapparatus comprising: a plurality of thin ferromagnetic strips that haveresidual magnetization therein, said thin ferromagnetic strips beingcoupled in parallel to said structure; a transmitter coil being locatedadjacent to a first of said thin ferromagnetic strips; a receiver coilbeing located adjacent to a second of said thin ferromagnetic strips; atransmitter control circuit connected to said transmitter coil forgenerating a pulse signal and delivering said pulse signal to saidtransmitter coil, said transmitter coil creating magnetostrictively aguided wave that is coupled from said first thin ferromagnetic strip tosaid structure to propagate along said structure; said receiver coilmagnetostrictively detecting said guided wave and any reflected signals,including any caused by defect or transient stress signals in saidstructure; said transmitter coil and said receiver coil being woundadjacent said first and second thin ferromagnetic strips, respectively,said guided waves moving perpendicular to said first and second thinferromagnetic strips.
 13. An apparatus for nondestructive, short terminspection or long term monitoring of a structure as recited in claim 12wherein said residual magnetization is in a lengthwise direction of saidthin ferromagnetic strips and said guided wave is a shear wave.
 14. Anapparatus for nondestructive, short term inspection or long termmonitoring of a structure as recited in claim 13 further including acomputer for storing a first of said reflected signals and, afterappropriate periods of time, comparing new reflected signals with saidstored reflected signal to determine if changes have occurred.
 15. Anapparatus for nondestructive, short term inspection or long termmonitoring of a structure as recited in claim 14 wherein a couplant forsaid coupling of said plurality of said thin ferromagnetic strips is abonding material, such as epoxy.
 16. An apparatus for nondestructive,short term inspection or long term monitoring of a structure as recitedin claim 12 wherein a couplant for said coupling of said plurality ofsaid thin ferromagnetic strips is a thick, viscous material, such ashoney.
 17. An apparatus for nondestructive, short term inspection orlong term monitoring of a structure as recited in claim 12 wherein saidplurality of thin ferromagnetic strips retain said residualmagnetization for a long period of time, such as nickel, grain-orientedsilicon steel, or TERFENDOL®.
 18. An apparatus for nondestructive, shortterm inspection or long term monitoring of a structure as recited inclaim 15 wherein at least four of said thin ferromagnetic strips areused, two for transmitting and two for receiving, so that direction oftravel of said guided wave can be determined, therefore only saidreflected signals in a given direction being stored in said computer.19. An apparatus for nondestructive, short term inspection or long termmonitoring of a structure recited in claim 15 wherein said transmittercoil and said receiver coil are coils on a flexible printed circuitboard which generates or receives said guided wave in said ferromagneticstrips.